The present invention relates to an improvement in the structure of an information recording medium on which address information, header information, moving picture information, audio information, and any other information are recorded.
More particularly, the present invention relates to an improvement in a high-density optical disk obtained by adhering two transparent substrates on which various kinds of information are recorded as embossed prepits, wherein at least some of information recording layers having prepits are made of a material having a higher refractive index than that of each substrate of the double-layered optical disk.
In multimedia applications for processing image information in a large quantity, to meet complicated needs and a great increase in quantity of necessary information, strong demand has arisen for a means for holding information in a larger size and allowing quick information access. A typical example of such an information holding means is an optical disk.
The most popular media belonging to the optical disks are a compact disk (CD) developed as a recording medium for music, a CD-ROM developed as a recording medium for computer data, and a laser disk (LD) developed as a recording medium for movies (moving pictures). These optical disks may have difficulty coping with future multimedia applications.
New types of media which can cope with the needs for large capacities and the high-speed, flexible information access are next-generation main recording media which are about to be on the market, i.e., a digital versatile disk and a digital video disk (to be referred to as a DVD hereinafter).
The capacity of one DVD disk standard, i.e., the capacity of a double-layered DVD having 0.6-mm thick transparent substrates (polycarbonate) is about 10 Gbytes on the two faces. Each face has a capacity of about 5 Gbytes. A 135-minute movie can be compressed on one face in accordance with the MPEG (Moving Picture Expert Group) standards.
A DVD disk employs a double-layered structure. For this reason, to read all the information on the two faces of the disk, the disk must be turned over at the end of a read operation on one face or a laser pickup (read optical head) must be moved from the upper face to the lower face of the disk, judging from the operation of a conventional optical disk (e.g., an LD). A user must turn over the disk at the end of a read operation on one face, resulting in inconvenience. Alternatively, an optical head for reading information from the two faces must be arranged to complicate the arrangement of a read apparatus. That is, a compact read apparatus is difficult to obtain, and the product cost becomes high, thus posing a problem.
To solve the above problem (i.e., the disk must be turned over, or the complicated two-face read mechanism is required), the following structure is employed in a DVD disk. More specifically, a semi-transparent film is used as a recording layer (first recording layer) on the read laser receiving face (upper face) facing an optical head and information on a recording layer (second recording layer) on the face (lower face) opposing the optical head can also be read. With this structure, the optical disk is always located on one face (upper face) side of the disk. The read face is switched by changing the focal point of the optical head between the first and second recording layers.
In this case, the transmittance of the first recording layer (transparency to the read laser beam) on the upper face side must be high to properly read information from the second recording layer as the lower face. The reflectance of the first recording layer (reflectance with respect to the same read laser beam as the transmitted laser beam) must be high.
Normally, a high transmittance contradicts a high reflectance with respect to the same laser beam. The number of transmission/reflection materials which balance the above contradictory factors on a practical level is limited. An example of such a material is a very thin gold film. When the first recording layer on the upper face side of the optical disk is made of a very thin gold film, it is difficult to assure the mechanical strength at this gold film portion because two disk substrates are adhered to each other through the very thin gold film. That is, when a user drops a double-layered disk in which the very thin gold film is present on the adhesion face, the two substrates of the double-layered disk may peel at this gold film portion due to a shock. Even in the absence of a dropping shock, the two substrates of the double-layered disk may peel at the thin gold film portion after long-term storage or in a high-temperature, high-humidity accelerated degradation test. In addition, since gold is an expensive material, it is disadvantageous to employ a thin gold film due to the disk manufacturing cost.